1. Field of the Invention
This invention relates to the field of wellbore logging and, more particularly, to a method and apparatus for determining nuclear magnetic resonance logging characteristics of earth formations surrounding a wellbore, as a function of angular position about the borehole, either during the drilling of the wellbore or after drilling.
2. Description of Related Art
Hydrocarbon fluids, such as oil and natural gas, are obtained from a subterranean geologic formation, referred to as a reservoir, by drilling a wellbore that penetrates the hydrocarbon-bearing formation. An understanding of the reservoir physical properties, often referred to as formation evaluation, is needed to determine the well""s productive capacity, recoverable reserves, size and type of production equipment needed, and many other issues relating to the well""s drilling, completion and production. Specific reservoir properties that are desired include, for example, porosity, permeability, and water saturations.
Electrical xe2x80x9cloggingxe2x80x9d dates back to 1912, when Conrad Schlumberger began studying the problem of exploring the underground by means of surface electrical measurements. Since that time, various types of well logging techniques have been developed, such as acoustic, temperature, resistivity, nuclear, and gamma-ray measurement techniques.
Nuclear magnetic resonance (NMR) imaging involves the physical principle that various nuclei will precess at different frequencies in an imposed magnetic field. Many nuclei have a magnetic moment and behave much like a spinning bar magnet. This spinning motion is often referred to in the NMR terminology as the particle spin and is illustrated in FIG. 27. Hydrogen nuclei, consisting of a single proton, has a relatively large magnetic moment and is found in both water and hydrocarbons that are located within a reservoir matrix. An externally applied magnetic field can interact with the spinning hydrogen protons and can produce measurable effects. An NMR logging tool can be designed to operate at the magnetic resonant frequency of hydrogen, thereby allowing the tool to alter and detect the responses of the hydrogen protons within the region of investigation. By altering and detecting responses, the tool can obtain information relating to the water and hydrocarbons within the reservoir.
A static magnetic field is generated by the tool to initially align the hydrogen protons in the formation fluids. An oscillating radio-frequency magnetic field is generated by the tool to alter the hydrogen protons alignment and tip the protons in a transverse plane. The tipped protons move in a precessional motion around the initial alignment position in a manner similar to a spinning top that precesses in the Earth""s magnetic field, as illustrated in FIG. 27. Various NMR measurements of these and other related effects can provide an indication of the amount of total fluid contained within the formation, and can be used to indicate the identity of the fluid, whether water, gas or oil. The measurements can also provide indications on the pore and grain size distribution of the formation matrix and whether the fluids are bound within the formation matrix or are capable of movement, and therefore, potentially producible.
One approach to obtaining nuclear magnetic resonance measurements involves inserting a NMR tool within the wellbore and applying a locally generated static magnetic field Bo, which can be produced by one or more electromagnets or permanent magnets. The spins of the hydrogen protons within the formation matrix near the tool are aligned with the applied field Bo, generating a net nuclear magnetization as the spinning hydrogen protons precess about the imposed magnetic field Bo, as illustrated in FIG. 28. Nuclear spins of the hydrogen protons align with the applied field Bo, generating a net nuclear magnetization. Applying an RF field, B1, perpendicular to the static field Bo, as illustrated in FIG. 29, can change the angle between the nuclear magnetization and the applied field Bo. The frequency of the RF field should be equal to the Larmor frequency given by xcfx890=xcex3Bo where xcex3 is the gyromagnetic ratio. After application of an RF pulse, the magnetization begins to precess around Bo and produces a detectable signal in the antenna. As the protons precess about the static field Bo, they gradually lose synchronization with each other, as illustrated in FIG. 30. This loss of synchronization causes the magnetic field in the transverse plane to decay. Phase encoding is caused by inhomogeneities in the static magnetic field and by molecular interactions. The signals can be analyzed to detect nuclear magnetic resonance properties of the formation and provide information relating to porosity, free fluid ratio, permeability, and other properties of the formation. See U.S. Pat. No. 4,717,878 issued to Taicher et al. and U.S. Pat. No. 5,055,787 issued to Kleinberg et al.
Another approach to obtaining nuclear magnetic resonance measurements employs a locally generated static magnetic field, Bo, which may be produced by one or more permanent magnets or electromagnets, and an azimuthally-oriented oscillating magnetic field, B1, which may be produced by one or more RF antenna segments that transmit and/or receive from different circumferential sectors of the logging device. See U.S. Pat. Nos. 5,977,768 and 6,255,817 assigned to Schlumberger Technology Corporation.
U.S. Pat. No. 5,796,252 issued to Kleinberg et al. describes a nuclear magnetic logging device that includes permanent magnets, an RF antenna, and a coil for generating a magnetic field gradient. The technique described in the ""252 patent utilizes pulsed magnetic field gradients to obtain information regarding diffusion properties of the formation fluids. If internal gradients are present in the formation, a pulse sequence is applied to reduce or substantially eliminate the effect of internal gradients in the formation. The ""252 patent does not identify a method for using the coil to obtain an azimuthal NMR measurement.
U.S. Pat. No. 5,212,447 issued to Zvi Paltiel describes a nuclear magnetic logging device that includes permanent magnets and an RF antenna coil. The ""447 patent requires a magnetic field gradient coil to determine a diffusion coefficient, i.e., the rate at which molecules of a material randomly travel within the bulk of the same material. The ""447 patent employs the diffusion coefficient to determine at least one of the following petrophysical parameters: water/hydrocarbon discrimination, water and hydrocarbon saturation levels, permeability, pore size and pore size distribution, oil viscosity, a measure of the average increase in electrical resistance due to the formation tortuosity, and q-space imaging of the formation. The ""447 patent does not identify a method for using the coil to obtain an azimuthal NMR measurement.
U.S. Pat. No. 6,326,784 assigned to Schlumberger Technology Corporation, discloses a means to obtain azimuthal NMR measurements using one or more gradient coils and an axi-symmetric antenna. In this approach, a pulse sequence and a firing of a gradient coil is used in conjunction to obtain an azimuthal image. The resolution of the azimuthal image depends on the number of gradient coils used and the angular coverage of each gradient coil. The gradient coils are positioned circumferentially and are separated by an angular distance, for example, three gradient coils located around an NMR tool spaced 120xc2x0 from each other. Each gradient coil is used to spoil or rotate the hydrogen proton spins within the formation matrix adjacent to the gradient coil, with negligent effects everywhere else. The NMR data obtained after the pulse sequence and firing of a gradient coil is used to obtain formation evaluation information of the reservoir adjacent to the gradient coil. This process is repeated for the other gradient coils to obtain the azimuthal image.
One method for acquiring azimuthal data is the binning of the measured data. In this acquisition scheme, a plurality of azimuthal bins is defined and each NMR measurement is added to the content of a buffer associated with the bin in which the measurement was taken. To obtain a good statistical quantity of data for each bin and to ensure that in each bin there are enough measurements, this scheme requires numerous individual measurements. For example, for one particular embodiment having a 90xc2x0 resolution, seven bins are preferable, with ten measurements per bin, or 70 measurements per scan. Better resolution, (i.e., smaller angular distances between gradient coils), would need substantially more measurements and result in substantially more time required to obtain the quantity of data needed.
There is a need for improved means to obtain NMR azimuthal images of a formation. There is also a need for greater azimuthal resolution of the formation properties, along with means to obtain this information with less acquisition time.
One embodiment of the present invention is a method for nuclear magnetic resonance imaging of an investigation region of formation surrounding a wellbore. The method comprises the steps of applying a series of magnetic field gradients to phase encode nuclei spins within the investigation region, wherein the strength of the magnetic field gradient applied for each spin-echo signal is different from at least one previously applied magnetic field gradients within the series. Nuclear magnetic resonance signals are detected from the investigation region resulting from the series of magnetic field gradients.
Another embodiment of the invention is an apparatus for determining a nuclear magnetic resonance property in an investigation region of earth formations surrounding a wellbore. The apparatus comprises a logging device moveable through the wellbore and a means in the logging device for applying a static magnetic field circumferentially around the wellbore and into the investigation region. The apparatus further comprises an antenna means in the logging device for applying an RF magnetic field circumferentially around the borehole and into the investigation region, whereby the antenna means induces a plurality of pulse echoes and spin-echo signals from selected nuclei of the formation. A gradient means in the logging device is capable of producing a gradient magnetic field circumferentially around the borehole and into the investigation region and is capable of producing different strength gradient magnetic fields for each of the plurality of pulse echoes, wherein the orientation of the gradient magnetic field and the static magnetic field effects on the selected nuclei vary depending on the azimuthal position around the wellbore in relation to the gradient means and means for detecting nuclear magnetic resonance signals from the investigation region.